Estimation Of Intentional Phase Shift In a Calibration Apparatus

ABSTRACT

Embodiments of the present invention provide an apparatus comprising a transceiver having a receiver and a transmitter connected through a segment of a calibration loop back path. The apparatus also comprises a control system configured to communicate with the transceiver. The calibration loop back path has an intentional phase shift that can be toggled between an off state and an on state by the control system. The control system is configured to calculate the intentional phase shift by examining the difference of a first and second phase angle. The first phase angle is obtained from the transmission of a first pair of signals with the intentional phase shift in the off state. The second phase angle is obtained from the transmission of a second pair of signals with the intentional phase shift in the on state.

RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.12/985,201, entitled “ESTIMATION OF INTENTIONAL PHASE SHIFT IN ACALIBRATION APPARATUS” filed Jan. 5, 2011.

FIELD OF THE INVENTION

This invention relates to radio transceivers, and more specifically tomethods and systems for calibrating radio transceivers.

BACKGROUND OF THE INVENTION

At the turn of the 20th century the airwaves of the United States werecompletely silent. Today they are filled to the brim with wirelesscommunication signals. The recent history of wireless telegraphy ischaracterized by a concerted push towards fitting more and moreinformation into the air. Complex modulation schemes have been developedthat package information with increasing efficiency. To match this everincreasing degree of efficiency radios are required to resolve thesesignals with a commensurate degree of precision. Precision is requiredbecause wireless signals are corrupted by noise as they are sent througha wireless medium and because as more complex patterns are used torepresent information it becomes more challenging to determine onepattern from another.

Modulation schemes such as Gaussian frequency shift keying (GFSK), Phaseshift keying (PSK), and Quadrature amplitude modulation (QAM) canutilize two channels that transmit signals that are 90° out of phasewith each other. The two channels are called the i-channel and theq-channel. These channels carry the in-phase and quadrature-phase signalrespectively. The signal to be transmitted is mixed with an in-phaselocal oscillator and a quadrature-phase local oscillator to form themodulated signals at a frequency that is amenable to transmissionthrough the air. Due to the principles of linearity, the two signals canbe sent through the air simultaneously and then the received signal canbe resolved at the receiver into the original component parts. Thismodulation scheme results in a highly efficient usage of availablebandwidth.

Although I/Q channel modulation has certain benefits regarding theamount of bandwidth consumed, the increased complexity of the modulationscheme can result in several errors that make resolving the signaldifficult. Like other wireless systems, I/Q channel modulated signalscan suffer from carrier leak. In order to effectively transmit andresolve a signal that has been mixed with a local oscillator signal thetransmitter and receiver must agree on what frequency the signal wasmixed with. If the two disagree on this frequency the receiver willmistakenly interpret a portion of the mixed frequency as the informationsignal. This phenomenon is called carrier leak. In addition, I/Q channelmodulated signals are susceptible to a group of errors that cancollectively be referred to as IQ imbalance errors. I/Q channelmodulation requires the receiver and transmitter to agree not only onwhat the carrier frequency is, but also on what the phase difference ofthe two signals are. If the transmitter does not impart the right amountof phase shift, or the receiver resolves the signals as if they had adifferent phase shift, this mismatch will also bleed into the signalitself and will show up as an error in the signal. The combined resultsof these errors can be broken up and expressed as four quantities; thephase error of the transmitter (Δθ_(T)), the phase error of the receiver(Δθ_(R)), the amplitude error of the transmitter normalized to nominalgain (ε_(T)), and the amplitude error of the receiver normalized tonominal gain (ε_(R)). The quantities can be collectively referred to asthe IQ imbalance error metrics of the transceiver.

Since manufacturing processes are not perfect, the errors described inthe previous paragraph will be different from part to part. This meansthat two parts that come off of the assembly line one after the othermay have manufacturing imperfections that result in highly variant errormetrics. Therefore, a designer cannot design out these error sources andproduce a design that will always work. Instead, wireless radios need tobe designed such that they can be individually calibrated to removethese errors.

SUMMARY OF INVENTION

In one embodiment of the invention, an apparatus is provided. Theapparatus comprises a transceiver having a receiver and a transmitterconnected through a segment of a calibration loop back path. Theapparatus also comprises a control system configured to communicate withthe transceiver. The calibration loop back path has an intentional phaseshift that can be toggled between an off-state and an on-state by thecontrol system. The control system is configured to calculate theintentional phase shift by examining the difference of a first andsecond phase angle. The first phase angle is obtained from a firsttransmission of a first pair of signals with the intentional phase shiftin the off-state. The second phase angle is obtained from a secondtransmission of a second pair of signals with the intentional phaseshift in the on-state.

In another embodiment of the invention, a method for estimating theintentional phase shift of a calibration loop back path is provided. Inone step a first and second signal are transmitted on a single channelof a transmitter along a segment of the calibration loop back path. Thecalibration loop back path connects the transmitter to a receiver andhas an intentional phase shift. In another step, the first and secondsignals are received with said receiver through the segment of thecalibration loop back path to obtain a quantum of information. Inanother step, a first phase angle is determined based on a quantum ofinformation. In another step, the intentional phase shift is activated.In another step, the third and fourth signal are transmitted on thesingle channel of the transmitter along the segment of the calibrationloop back path. In another step, a third and fourth signals are receivedwith the receiver through the segment of the calibration loop back pathto obtain a second quantum of information. In another step a secondphase angle is determined base on the second quantum of information. Inanother step, the intentional phase shift is calculated based on thefirst and second phase angles to obtain an estimated value of theintentional phase shift.

In another embodiment of the invention, a system is provided. The systemcomprises a transceiver having a receiver and a transmitter connectedthrough a segment of a calibration loop back path. The system alsocomprises a control system configured to communicate with thetransceiver. The system also comprises an IQ mismatch calibration unitthat is within the control system, and a transmitter pre-distortion unitthat is also within the control system. The transmitter pre-distortionunit is configured to provide a pre-distortion compensation. Thecalibration loop back path has an intentional phase shift that can betoggled between an off-state and an on-state by the control system. Thecontrol system is configured to calculate the intentional phase shift byexamining the difference of a first and second phase angle. The firstphase angle is obtained from the transmission of a first pair of signalswith the intentional phase shift in the off-state. The second phaseangle is obtained from the transmission of a second pair of signals withthe intentional phase shift in the on-state. The IQ mismatch calibrationunit calculated the pre-distortion compensation based on a set ofmeasurements obtained using the intentional phase shift.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a block diagram of an apparatus that is in accordancewith the present invention.

FIG. 2 illustrates a block diagram of a phase shift unit that can beused in accordance with the present invention.

FIG. 3 illustrates a process flow chart of a method for determining theintentional phase shift of an apparatus that is in accordance with thepresent invention.

FIG. 4 illustrates signals transmitted and received using I/Q channelmodulation during a process that is in accordance with the presentinvention.

FIG. 5 illustrates a block diagram of a system that is in accordancewith the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Reference will now be made in detail to embodiments of the presentinvention, examples of which are illustrated in the accompanyingdrawings. While the invention will be described in conjunction withthese embodiments, it will be understood that they are not intended tolimit the invention to these embodiments. On the contrary, the inventionis intended to cover alternatives, modifications and equivalents, whichmay be include within the spirit and scope of the invention as definedby the appended claims. Furthermore, in the following detaileddescription of embodiments of the present invention, numerous specificdetails are set forth in order to provide a thorough understanding ofthe present invention. However, it will be recognized by one of ordinaryskill in the art that the present invention may be practiced withoutthese specific details. In other instances, well-known methods,procedures, components, and circuits have not been described in detailas not to unnecessarily obscure aspects of the embodiments of thepresent invention.

Some portions of the detailed descriptions which follow are presented interms of procedures, logic blocks, processing and other symbolicrepresentations of operations on data bits within a computer memory.These descriptions and representations are the means used by thoseskilled in the data processing arts to most effectively convey thesubstance of their work to others skilled in the art. In the presentapplication, a procedure, logic block, process, or the like, isconceived to be a self-consistent sequence of steps or instructionsleading to a desired result. The steps are those requiring physicalmanipulations of physical quantities. Usually, although not necessarily,these quantities take the form of electrical or magnetic signals capableof being stored, transferred, combined, compared, and otherwisemanipulated in a computer system.

It should be borne in mind, however, that all of these and similar termsare to be associated with the appropriate physical quantities and aremerely convenient labels applied to these quantities. Unlessspecifically stated otherwise as apparent from the followingdiscussions, it is appreciated that throughout the present application,discussions utilizing the terms such as “accessing,” “receiving,”“sending,” “using,” “selecting,” “determining,” “normalizing,”“multiplying,” “averaging,” “monitoring,” “comparing,” “applying,”“updating,” “measuring,” “deriving” or the like, refer to the actionsand processes of a computer system, or similar electronic computingdevice, that manipulates and transforms data represented as physical(electronic) quantities within the computer system's registers andmemories into other data similarly represented as physical quantitieswithin the computer system memories or registers or other suchinformation storage, transmission or display devices.

Embodiments described herein may be discussed in the general context ofcomputer-executable instructions residing on some form ofcomputer-usable medium, such as program modules, executed by one or morecomputers or other devices. Generally, program modules include routines,programs, objects, components, data structures, etc., that performparticular tasks or implement particular abstract data types. Thefunctionality of the program modules may be combined or distributed asdesired in various embodiments.

By way of example, and not limitation, computer-usable media maycomprise computer storage media and communication media. Computerstorage media includes volatile and nonvolatile, removable andnon-removable media implemented in any method or technology for storageof information such as computer-readable instructions, data structures,program modules or other data. Computer storage media includes, but isnot limited to, random access memory (RAM), read only memory (ROM),electrically erasable programmable ROM (EEPROM), flash memory or othermemory technology, compact disk ROM (CD-ROM), digital versatile disks(DVDs) or other optical storage, magnetic cassettes, magnetic tape,magnetic disk storage or other magnetic storage devices, or any othermedium that can be used to store the desired information.

Communication media can embody computer-readable instructions, datastructures, program modules or other data in a modulated data signalsuch as a carrier wave or other transport mechanism and includes anyinformation delivery media. The term “modulated data signal” means asignal that has one or more of its characteristics set or changed insuch a manner as to encode information in the signal. By way of example,and not limitation, communication media includes wired media such as awired network or direct-wired connection, and wireless media such asacoustic, radio frequency (RF), infrared and other wireless media.Combinations of any of the above should also be included within thescope of computer-readable media.

A specific embodiment of the invention can be described with referenceto FIG. 1. FIG. 1 illustrates a transceiver 100 having a receiver 101and a transmitter 102 connected through segment of a calibration loopback path 103. The entire calibration loop to which segment 103 is apart extends from control system 104, into transmitter 102, then intoreceiver 101, and then back into control system 104. Control system 104can communicate with transceiver 100 through analog-to-digital converter(ADC) 105 connected to receiver 101 and digital-to-analog converter(DAC) 106 connected to transmitter 102. Segment 103 is shown connectingthe intersection of variable gain amplifier 107 and power amplifier 108with the intersection of low noise amplifier 109 and variable gainamplifier 110. However, segment 103 can comprises any part of anycalibration loop described above. For example, segment 103 can comprisesa portion of a feedback loop that directly links mixer pairs 111 and112. The calibration loop is activated during a calibration phase of thecircuit and has an intentional phase shift that can be toggled betweenan off-state and an on-state by control system 104. In specificembodiments of the invention, transceiver 100 will utilize QAM. In theseembodiments mixer pairs 111 and 112 will mix in and demodulate the localfrequency to the information signal using both an in-phase andquadrature carrier frequency signal. Mixer pairs 111 and 112 may beconnected to ADCs and DACs that are physically separate circuits.

The intentional phase shift of the loop back path illustrated in FIG. 1can be used to calibrate transceiver 100. A process for executing thiscalibration is described in U.S. patent application Ser. No. (12/328,128filed on Dec. 4, 2008) to Alibeik et al which is incorporated byreference as if set for herein in its entirety. The process involvessending two sets of signals from transmitter 102 to receiver 101 alongloop segment 103. The first set of signals is sent with the intentionalphase shift in the off-state and the second set of signals is sent withthe intentional phase shift in the on-state. By resolving these two setsof signals it is possible to calculate all four of the IQ imbalanceerror metrics Δθ_(T), Δθ_(R), ε_(T), and ε_(R). Once these values areknown, their effects can be calibrated out by control system 104.Control system 104 will be able to calculate a compensation signal toadd to any information signal it sends with transmitter 102 to eliminateIQ imbalance errors from the transmitted signal. In addition, controlsystem 104 will be able to resolve signals it receives with receiver 101in accordance with a compensation scheme to eliminate IQ imbalanceerrors from the received signal. The calculation conducted by controlsystem 104 that solves for the IQ imbalance error metrics is dependentupon the intentional phase shift. As described in the above referencedapplication this value is treated as a constant.

The process described in the previous paragraph is an efficient processfor determining the IQ imbalance errors of transceiver 100 with minimalmeasurements. However, the process depends on the intentional phaseshift being a known value and which is not necessarily true. Theintentional phase shift is not easy to discern and does not always equalthe expected value. Just as the IQ imbalance errors are variable, theintentional phase shift changes from part to part. Therefore, theintentional phase shift should be measured before the IQ imbalancecalibration is carried out. Once the intentional phase shift is measuredit can be used as a constant in the IQ imbalance calibration. FIG. 2illustrates a phase shift generator that can be used in place of loopsegment 103. FIG. 2 illustrates circuit 200 that can be used inaccordance with the present invention to introduce a phase shift in theloop back path. The embodiments to which this configuration is inaccordance generate the intentional phase shift in loop segment 103, butin other embodiments of the invention the intentional phase shift isgenerated elsewhere in the calibration loop. Transistor 201 has itssource connected to ground and its drain connected to node 202. Node 202connects to the receiver. Transistor 201 has its gate connected toresistor 203 and switch 204. Node 205 connects to the transmitter.Circuit 200 is switched between the on-state and off-state by changingthe state of switch 204. By properly selecting resistor 203, circuit 200can add a desired phase shift to the loop back path. In specificembodiments of the invention this phase shift will be 90°. As mentionedpreviously, the intentional phase shift introduced by circuits such ascircuit 200 is very important for the loop back method calculation ofthe IQ imbalance errors. However, the phase shift introduced by circuit200 will depend on the actual resistance of resistor 203 and thecharacteristics of transistor 201. Since the value of the intentionalphase shift will vary from part to part it must be measured before theloop back method is executed to calculate the IQ imbalance errors.

Specific embodiments of the present invention are able to calculate theintentional phase shift of the loop back path of transceivers such astransceiver 100. In specific embodiments of the invention, controlsystem 104 is configured to calculate the intentional phase shift of theloop back path by examining the difference of a first and second phaseangle. The first phase angle is obtained from the transmission of afirst pair of signals through the loop back path with the intentionalphase shift unit in the off-state. The second phase angle is obtainedfrom the transmission of a second pair of signals through the loop backpath with the intentional phase shift in the on-state. In specificembodiments of the invention the first and second pairs of signals aretransmitted on the i-channel transmitter path of transmitter 102 and arereceived on the i-channel and q-channel receiver paths of receiver 101.In specific embodiments of the invention, the first and second pairs ofsignals are identical. In specific embodiments of the invention controlsystem 104 will comprises a coordinate rotation digital computer(CORDIC) that is configured to receive a quantum of informationregarding the first and second pairs of signals through ADC 105 and isconfigured to calculate the first and second phase angles based on thatquantum of information. In specific embodiments of the invention controlsystem 104 will be implemented in digital circuitry. In specificembodiments of the invention control system 104 will comprise or bepartially comprised of software.

A specific embodiment of the invention can be described with referenceto FIG. 3. FIG. 3 illustrates a method for estimating an intentionalphase shift of an IQ calibration apparatus that is in accordance withthe present invention. With reference to FIG. 1, this method can be usedto calculate the intentional phase shift of the calibration loop in atransceiver such as transceiver 100. In step 300 a first and secondsignal are transmitted on a single channel of a transmitter along asegment of a calibration loop back path that connects the transmitter tothe receiver. In step 301 the first and second signals are received bythe receiver through the segment of the calibration loop back path and aquantum of information is obtained from the received signals. In step302 a first phase angle is determined based on this quantum ofinformation. In step 303 an intentional phase shift is activated. Inspecific embodiments of the invention the phase shift will be producedby a circuit such as circuit 200 shown in FIG. 2 and will be positionedas loop segment 103 in FIG. 1. In step 304 a third and fourth signal aretransmitted on the same single channel of the transmitter along the samesegment of the calibration loop back path. In step 305 the third andfourth signals are received through this segment of the calibration loopback path and a second quantum of information will be obtained from thereceived signals. In step 306 a second phase angle is determined basedon the second quantum of information. In step 307 an estimate of theintentional phase shift is calculated based on the first and secondphase angles.

In specific embodiments of the invention, the receiver and transmitterused in step 300, 301, 304, and 305 can be components of a singletransceiver. In specific embodiments of the invention this transceiveris capable of functioning in accordance with a Bluetooth protocol. Inspecific embodiments of the invention, the method comprising steps300-307 can be executed using a control system such as control system104 described with reference to FIG. 1. For example, the steps ofdetermining the first and second phase angles can be executed by aCORDIC that forms a portion of control system 104.

In specific embodiments of the invention, the method can additionallycomprise the step of checking the magnitude of the average value of thefirst, second, third, and fourth signals at the receiver against asaturation protection threshold. The information obtained in this stepcan be used to readjust the measurement and calibration algorithm sothat the receiver is not saturated and the calibration signals are notresolved incorrectly by the receiver. In specific embodiments of theinvention if the saturation protection threshold is exceeded thereceiver baseband gain will be reduced. For example, the receiverbaseband gain may be reduced by 6 dB. As a result it is possible forevery transmission gain to be calibrated while keeping the signal at theADC the same.

In specific embodiments of the invention, the method can additionallycomprise the step of cancelling the DC offset of the receiver. This stepcould precede step 300 and could improve the accuracy of the determiningsteps 302 and 306. This step would be most useful when the receiver wasuncalibrated. If the receiver was designed to be extremely precise orwas calibrated through other means this step would not be necessary.

In specific embodiments of the invention, the receiving and transmittingin steps 300, 301, 304, and 305 can be executed in accordance with aBluetooth protocol. Modulation can be conducted using GFSK, PSK, QAM, orany other digital modulation scheme. The transmitter used fortransmitting could comprise an i-channel transmitter path and aq-channel transmitter path. The receiver could comprise an i-channelreceiver path and a q-channel receiver path. The operation of steps 300and 301 and steps 304 and 305 that are in accordance with theseembodiments can be described with reference to FIG. 4.

FIG. 4 displays three sets of axes 402 and 401, 404 and 403, and 405 and406. Axis 401 illustrates the magnitude of a signal sent on thei-channel of a transmitter. Axis 402 illustrates the magnitude of asignal sent on the q-channel of a transmitter. Therefore, signal 407 issent solely on the i-channel and has a magnitude that is equal butopposite in sign to that of signal 408. As such, signal 407 could be thefirst signal transmitted in step 300 where the signal is transmittedonly on the i-channel transmitter path and not on the q-channel.Likewise, signal 408 could be the second signal transmitted in step 300in a similar fashion but with opposite magnitude. Axes 403 and 405 bothillustrate the magnitude of a signal received on the i-channel of areceiver. Axes 404 and 406 both illustrate the magnitude of a signalreceived on the q-channel of a receiver.

FIG. 4 illustrates how IQ imbalances can result in the alteration of asignal from the transmitter to the receiver. On axis 401 and 402 it isshown how signal 407 is sent only on the i-channel. However, on axis 403and 404 it is shown how signal 407 is received partially on thei-channel and partially on the q-channel. The phase angle 411 can bedetermined by control circuitry in step 302 and 306. If the intentionalphase shift is activated and the same signals are resent such that thefirst and third signals and the second and fourth signals are identicalthe only difference in phase angle 411 will result from the addition ofthe intentional phase shift. Therefore subtracting the phase angle 411obtained in step 302 from the phase angle 411 obtained in step 306 willproduce an estimate of the intentional phase shift.

In specific embodiments of the invention the process described abovewith reference to FIGS. 3 and 4 can be executed by circuitry that servesanother purpose such that the method can be executed with very littlespecial purpose control circuitry or software. The control circuitrythat determines phase angle 411 can also be used to rotate signal 407and signal 408 about the origin by the phase angle 411. This process isused to determine the carrier leak because the inherent DC offset of thetransceiver can be measured by comparing the relative position ofsignals 407 and 408 on axes 402 and 401 with their resultant positionson axes 405 and 406. Since the circuitry involved with thesemeasurements serves a dual purpose the intentional phase shift can bemeasured with a minimal increase in the overall system's complexity.

A specific embodiment of the invention can be described with referenceto FIG. 5. Figure illustrates a system 500 that is in accordance withthe present invention. System 500 comprises a transceiver 501 having areceiver 502 and a transmitter 503 that are connected through a segmentof a calibration loop back path 504. In specific embodiments of theinvention transceiver 501 will operate in accordance with a Bluetoothprotocol. System 500 additionally comprises a control system 505 that isconfigured to communicate with transceiver 501. Control system 505 cancomprise digital circuitry. In specific embodiments of the invention,control system 505 will comprise or be partially comprised of software.In specific embodiments of the invention control system 505 is comprisedof a CORDIC. System 500 additionally comprises an IQ mismatchcalibration unit 506 that is within control system 505. System 500 alsocomprises a transmitter pre-distortion unit 507 within control system505 that is configured to provide a pre-distortion compensation tosignals that are to be sent by transmitter 503. The calibration loopback path to which loop segment 504 belongs has an intentional phaseshift that can be toggled between an off-state and an on-state bycontrol system 505. Control system 505 is configured to estimate thisintentional phase by executing a method that is in accordance with FIG.3. In specific embodiments of the invention IQ mismatch calibration unit506 will then calculate the pre-distortion compensation to be providedby pre-distortion unit 507 based in part on the intentional phase shiftestimate obtained by control system 505.

In specific embodiments of the invention, system 500 will suffer fromvarious analog impairments that affect receiver 502 and additionally oralternatively affect transmitter 503. These analog impairments caninclude phase noise, IQ imbalance impairments, and carrier leakimpairments. These analog impairments can be compensated bypre-distortion unit 507. In specific embodiments of the invention,pre-distortion unit 507 will compensate for these impairments bydigitally processing the signal to be sent by transmitter 503. Inspecific embodiments of the invention, pre-distortion unit 507 is ableto compensate for these impairments through the use of IQ mismatchcalibration unit 506, the execution of methods consistent with FIG. 3,and the intentional phase shift on the calibration loop.

The present invention can operate in accordance with a wirelesscommunication system. A wireless communication system may comprise aplurality of communication devices, such as a personal computer (PC), aportable device, a WLAN access point, server, and a wireless portablecommunication device. In specific embodiments of the invention, theplurality of communication devices comprise a WLAN device including atransceiver, such as transceiver 100, that is configured to transmit andreceive WLAN signals. In specific embodiments of the invention, theplurality of communication devices additionally comprise a PAN deviceincluding a transceiver, such as transceiver 100, that is configured totransmit and receive PAN signals.

Although embodiments of the invention have been discussed primarily withrespect to specific embodiments thereof, other variations are possible.Various configurations of the described system may be used in place of,or in addition to, the configurations presented herein. Those skilled inthe art will appreciate that the foregoing description is by way ofexample only, and is not intended to limit the invention. Nothing in thedisclosure should indicate that the invention is limited to systems thatfunctions with only a single transceiver or to two channelreceiver/transmitter pairs. Nothing in the disclosure should limit thescope of the invention to electronics, communication through the use ofcharged particles or electro-magnetic waves, or communication through awireless medium. Functions may be performed by hardware or software, asdesired. In general, any diagrams presented are only intended toindicate one possible configuration, and many variations are possible.As used in the specification and in the appended claims the term“quantum of information” refers to a unit of information that can be inany form and size so long as it is comprises resolvable coherentinformation. Those skilled in the art will also appreciate that methodsand systems consistent with the present invention are suitable for usein a wide range of applications encompassing any related tocommunications or information technology in general.

While the specification has been described in detail with respect tospecific embodiments of the invention, it will be appreciated that thoseskilled in the art, upon attaining an understanding of the foregoing,may readily conceive of alterations to, variations of, and equivalentsto these embodiments. These and other modifications and variations tothe present invention may be practiced by those skilled in the art,without departing from the spirit and scope of the present invention,which is more particularly set forth in the appended claims.

1. An apparatus comprising: a transceiver that includes a receiver, atransmitter and a calibration loop back path coupling the transmitter tothe receiver, wherein the calibration loop back path has a first stateand a second state, wherein no intentional phase shift is introduced tosignals transmitted through the calibration loop back path in the firststate, and wherein an intentional phase shift is introduced to signalstransmitted through the calibration loop back path in the second state;and a control system coupled to the transceiver, wherein the controlsystem is configured to identify the intentional phase shift bydetermining a difference between a first phase angle and a second phaseangle, wherein the first phase angle is obtained in response totransmitting a first pair of signals from the transmitter to thereceiver_through the calibration loop back path while the calibrationloop back path is configured in the first state, and the second phaseangle is obtained in response to transmitting a second pair of signalsfrom the transmitter to the receiver through the calibration loop backpath while the calibration loop back path is configured in the secondstate.
 2. The apparatus of claim 1, wherein the control system controlswhether the calibration loop back path is configured in the first stateor the second state.
 3. The apparatus of claim 1, wherein thecalibration loop back path is designed to introduce an intentional phaseshift of 90 degrees to signals transmitted through the calibration loopback path in the second state.
 4. The apparatus of claim 1, wherein thecalibration loop back path is coupled between a first variable gainamplifier in the transmitter and a second variable gain amplifier in thereceiver.
 5. The apparatus of claim 1, wherein the calibration loop backpath includes a resistor that introduces the intentional phase shift tosignals transmitted through the calibration loop back path in the secondstate.
 6. The apparatus of claim 1, wherein the control system includesa coordinate rotation digital computer (CORDIC) configured to receive aquantum of information associated with the first pair of signalstransmitted through the calibration loop back path and the second pairof signals transmitted through the calibration loop back path, anddetermine the first phase angle and the second phase angle in responseto the quantum of information.
 7. The apparatus of claim 1, wherein thetransceiver is configured to function in accordance with a Bluetoothprotocol.
 8. The apparatus of claim 1, wherein the transceiver furthercomprises: a digital-to-analog converter (DAC) coupling the controlsystem to the transmitter; and an analog-to-digital converter (ADC)coupling the receiver to the control system.
 9. The apparatus of claim1, wherein the transmitter comprises an in-phase channel (i-channel)transmitter path, and the receiver comprises an i-channel receiver pathand a quadrature-channel (q-channel) receiver path, wherein the controlsystem configures the transceiver to transmit the first pair of signalsand the second pair of signals through the i-channel transmitter path,to the calibration loop back path, to the i-channel receiver path andthe q-channel receiver path.
 10. The apparatus of claim 1, wherein thefirst pair of signals is identical to the second pair of signals. 11.The apparatus of claim 1, wherein the control system comprises: atransmitter pre-distortion unit that provides pre-distortioncompensation to signals provided from the control system to thetransmitter; and a calibration unit that determines the pre-distortioncompensation in response to the identified intentional phase shift. 12.A method comprising: configuring a calibration loop back path to have afirst state, wherein the calibration loop back path introduces nointentional phase shift in the first state; transmitting a first signaland a second signal from a transmitter to a receiver through thecalibration loop back path while the calibration loop back path is inthe first state, thereby providing a first quantum of information at thereceiver; determining a first phase angle in response to the firstquantum of information; configuring the calibration loop back path tohave a second state, wherein the calibration loop back path introducesan intentional phase shift in the second state; transmitting a thirdsignal and a fourth signal from the transmitter to the receiver throughthe calibration loop back path while the calibration loop back path isin the second state, thereby providing a second quantum of informationat the receiver; determining a second phase angle in response to thesecond quantum of information; and identifying the intentional phaseshift by determining a difference between the first phase angle and thesecond phase angle.
 13. The method of claim 12, further comprisingtransmitting the first, second, third and fourth signals on a singlechannel within the transmitter.
 14. The method of claim 13, furthercomprising receiving the first, second, third and fourth signals on apair of channels within the receiver.
 15. The method of claim 12,further comprising: determining a magnitude of an average value of thefirst, second, third and fourth signals at the receiver; comparing themagnitude of the average value with a threshold; and reducing a gain ofthe receiver if the magnitude of the average value exceeds thethreshold.
 16. The method of claim 12, further comprising implementing aGaussian frequency shift keying (GFSK) modulation scheme with thetransmitter and the receiver.
 17. The method of claim 12, wherein thefirst signal is identical to the third signal, and the second signal isidentical to the fourth signal.
 18. The method of claim 12, furthercomprising designing the calibration loop back path to introduce anintentional phase shift of 90 degrees to signals transmitted through thecalibration loop back path in the second state.
 19. An apparatuscomprising: a transceiver that includes a receiver, a transmitter and acalibration loop back path coupling the transmitter to the receiver;means for configuring the calibration loop back path to have a firststate and a second state, wherein no intentional phase shift isintroduced to signals transmitted through the calibration loop back pathin the first state, and wherein an intentional phase shift is introducedto signals transmitted through the calibration loop back path in thesecond state; means for transmitting a first pair of signals from thetransmitter to the receiver through the calibration loop back path whilethe calibration loop back path is in the first state, thereby providinga first quantum of information at the receiver; means for determining afirst phase angle in response to the first quantum of information; meansfor transmitting a second pair of signals from the transmitter to thereceiver through the calibration loop back path while the calibrationloop back path is in the second state, thereby providing a secondquantum of information at the receiver; means for determining a secondphase angle in response to the second quantum of information; and meansfor identifying the intentional phase shift in response to the firstphase angle and the second phase angle.